(1) Field of the Invention
The present invention relates to acoustic sensors and more particularly to broadband, acoustically transparent, nonresonant, passive PVDF hydrophones.
(2) Description of the Prior Art
Conventional hydrophones are made of piezoelectric materials that are acoustically hard (having a large characteristic acoustic impedance, i.e., density sound speed product, .rho.c) compared to the surrounding water medium with impedances 10 to 20 times that of water. Because of this acoustic impedance mismatch, an incoming sound wave is partiall reflected from and diffracted around the hydrophone. The pressure sensed by the hydrophone is thus not the free field pressure but the sum of the free field and the diffracted pressures. Because the latter depend on the frequency, they give rise to a frequency-dependent hydrophone sensitivity response. Furthermore, the mechanical vibrations induced in the piezoelectric element by the sound pressure field undergo strong internal reflections at the element boundaries because of the impedance mismatch between the element and the acoustic medium. This means that the element is resonant at certain frequencies, with a response that can be 10 dB or so larger than at other frequencies. Of course one usually operates the hydrophone at frequencies well below these resonances. It is not always practical however to eliminate small components (such as harmonics of the frequencies of interest) near resonance that become unduly amplified by the hydrophone response.
Piezoelectric polyvinylidene fluoride (PVDF) material approaches water's acoustic impedance, having a characteristic impedance of about 2.7 times that of water. This material was, however, available only in thin, nonvoided sheets having very low sensitivities. In order to provide adequate hydrophone sensitivity such material would have to be combined with pressure-release components such as compliant tubes or cylinders which would then reintroduce reflection problems. U.S. Pat. No. 4,433,400 describes an acoustically transparent hydrophone which utilizes such nonvoided, thin-film, PVDF sheets stretched over a metal hoop. The "transparency" in this case is due only to the fact that the PVDF sheets are very thin (.about.50 .mu.m). This type of hydrophone has very low sensitivity (.about.-234 dB//1 V/.mu.Pa) and exhibits resonances at frequencies below 1 MHz due to the presence of the hoop.
Thorn EMI Central Research Laboratories has developed a process for producing voided PVDF. Voided PVDF is produced by tensile drawing PVDF material in a manner which induces microcavities throughout the film. Tensile drawing of the material is carried out under conditions of high stress. The high stress is achieved by drawing the material at relatively low temperatures and high speeds in order to produce the microcavities, e.g., 80.degree. C. and 55 mm/minute. This material has been produced in thicknesses up to 1 mm and does not require the use of pressure-release components because it can be operated in a volume-expander mode. As a result of the voiding process the characteristic impedance can actually be made as low as 85% that of water.